Electrodeposition of alloys containing copper and tin



United States Patent LECTRQDE OSI ION OF ALLOYS CONTAI N COPPER AND N Erich Heymann and Grigory Schmerling,

' London, England Application November 14, 1952, Serial No. 320,617

Claims priority, application Great Britain emb r 1951 7 Claims. (Cl. 204-44) No Drawing.

The present invention relates to improvements in electroplating and is more particularly concerned with the electrodeposition of alloys containing copper and tin as the main constituents.

As is well known in bright chromium-plating techniques, the usual practice is to deposit a thin film of chromium on an electrolytically-deposited under-coating of another metal, conveniently nickel. As an alternative which affords many technical advantages, the use of an alloy containing copper and tin as the main constituents in place of nickel has much to recommend it, but difliculties have hitherto been encountered in obtaining a satisfactory deposit under production conditions without undue complications. Thus the use of alloy anodes causes polarization and other difficulties and the necessary provision of separate copper and tin anodes, used either simultaneously in separate circuits or alternately, has meant added complications and cost.

The object of the present invention is to provide a process whereby an alloy containing copper and tin as the main constituents may be deposited on a cathode with no more difficulty than is encountered in the electrodeposition of nickel or pure copper with substantially the same equipment.

According to the invention, the difficulties hitherto encountered in electro-depositing an alloy containing copper and tin as the main constituents, using anodes of the alloy in question, are overcome by including in the aqueous electrolyte an organic hydroxy acid, preferably citric acid, or a suitable salt thereof. When this is done, the alloy anodes dissolve in the bath in a steady and regular manner without any tendency to polarization or other disturbances. Moreover, the presence of citric acid or its compounds leads to a marked improvement in the appearance and texture of the deposited alloy.

An example of a suitable composition for the electrolyte is as follows:

Example 1 Gm./1. Copper cyanide 40 Sodium stannate 20 Sodium cyanide (total) 65 Sodium hydroxide 7 /2 Sodium citrate 50 This gives a rather weak electrolyte and in circumstances in which the use of a more concentrated electrolyte is preferable, the following composition might be used:

The quantities of the various components may vary within the ranges defined by the upper and lower limits set forth in the two preceding examples.

For n tan e, t p oportions by w igh of th s lids m yivary as foll s;

Other suitable examples of electrolyte compositions are as follows:

Example 3 Preferred Percent- Concept Qon em e fi ion tration, Range, Range, g./1. percent g./l.

copper earbonate 55 7-60 20-80 potassium stannate 30 3-75 10-50 sodium cyanide (total) 65 7-60 20-80 sodium hydroxide 5 0. 25-20 1-20 potassium citrate 3-70 25-125 Example 4 Preferred Percent- Concen- Concenage tration tration, Range, Range, g./l. percent g./l.

Example 5 Preferred Percent- Concen- Concenage tration tration, Range, Range, g./l. percent g./l.

copper sulphitel 50 7-50 20-80 sodium cyanide (total)... 50 7-50 20-80 sodium stannate 60 3-40 10-50 potassium hydroxide 50 10-40 20-100 citric acid 50 10-40 20-100 The various components may be added to the aqueous bath separately or they may be mixed in suitable proportions beforehand and subsequently dissolved in the appropriate amount of water.

The composition of the anodes, which primarily controls the nature of the deposit, may be -95% copper and 520% tin and very good results have been obtained with copper and 10% tin. It has also been found advantageous to include a small proportion of aluminum or aluminum alloy, for instance up to 2.5% of aluminum or up to 3% of an alloy of aluminum and magnesium.

It will be understood that although the deposit obtained according to the invention has been suggested as an undercoating for subsequent chromium plating, it can also be used as an independent finish or as a base for a further plating operation involving the deposit of other metals.

We claim:

1. A liquid for use as the electrolyte in an electrolytic bath for the deposition of a copper-tin alloy selected from the group consisting of an alloy of copper and tin, an alloy of copper, tin and up to 2.5% aluminum, and an alloy of copper, tin and up to 3% of an aluminum and magnesium alloy from anodes of the alloy to be deposited, said liquid composed of water, having dissolved therein 5-25% copper cyanide, 10-50% sodium stannate, 1035% sodium cyanide, 35% sodium hydroxide and 1045% sodium citrate.

2. A composition adapted for use when dissolved in water as the electrolyte in an electrolytic bath for the deposition of a copper-tin alloy selected from the group consisting of an alloy of copper and tin, an alloy of copper, tin and up to 2.5% aluminum, and an alloy of copper, tin and up to 3% of an aluminum and magnesium alloy from anodes of the alloy to bedeposited, said composition consisting of 5-25 copper cyanide, 1050% sodium stannate, 1035% sodium cyanide, 35% sodium hydroxide and 10-45% sodium citrate.

3. A process for the electrodeposition of a copper-tin alloy selected from the group consisting of an alloy of copper and tin, an alloy of copper, tin and up to 2.5 aluminum, and an alloy of copper, tin and up to 3% of an aluminum'and magnesium alloy on a cathode comprising passing an electric current through an aqueous electrolyte composed of water, 5-25 of a copper compound, 10-50% of an alkali-metal stannate, 10-35% of an alkali-metal cyanide and 1050% of a compound having the citric acid radical selected from the group consisting of citric acid and an alkali-metal salt of citric acid,

and in which electrolyte the cathode and an anode of the alloy to be deposited are immersed.

4. A process as claimed in claim 3 in which the compound in the electrolyte is sodium citrate.

5. A process as claimed in claim 3 in which the compound in the electrolyte is an alkali-metal citrate.

6. A process as claimed in claim 3 in which the compound in the electrolyte is citric acid.

7. A process as claimed in claim 3 in which the composition of the anode is 80-95% copper, 520% tin, 0-2.5% aluminum and 0-2% magnesium.

References Cited in the file of this patent UNITED STATES PATENTS 1,073,432 Marino Sept. 16, 1913 1,863,869 McCullough et al June 21, 1932 2,198,365 Cinamon et al Apr. 23, 1940 2,397,522 Baier Apr. 2, 1946 

1. A LIQUID FOR USE AS THE ELECTROYLTE IN AN ELECTROLYTIC BATH FOR THE DEPOSITION OF A COPPER-TIN ALLOY SELECTED FROM THE GROUP CONSISTING OF AN ALLOY OF COPPER AND TIN, AND ALLOY OF COPPER, TIN AND UP TO 2.5% ALUMINUM, AND AN ALLOY OF COPPER, TIN AND UP TO 3% OF AN ALUMINUM AND MAGNESIUM ALLOY FROM ANODES OF THE ALLOY TO BE DEPOSITED, SAID LIQUID COMPOSED OF WATER, HAVING DISSOLVED THEREIN 5-25% COPPER CYANIDE, 10-50% SODIUM STANNATE, 10-35% SODIUM CYANIDE, 3-5% SODIUM HYDROXIDE AND 10-45% SODIUM CITRATE. 